Re: performance of bitmap scans in nested loop joins

"Sergey E. Koposov" <math(at)sai(dot)msu(dot)ru> writes:
> And I coadded the "flat profiles" of first two (index scan) queries and
> compared it with the flat profile of bitmap scan:

Thanks, I had been thinking of doing that same calculation but hadn't
got round to it yet. It looks like the bitmap case is actually a little
ahead on buffer access (as you'd expect) and btree work (which is
surprising because it ought to be dead even; are these numbers very
repeatable?). Where we are losing is mostly on the actual manipulation
of the bitmaps (particularly hash_seq_search which is done in
tbm_begin_iterate; and it looks like memory allocation for the bitmap
hashtables is nontrivial too). I had already had a TODO item to look
into speeding up hash_seq_search ... will see what I can find.

regards, tom lane